1. Field of the Invention
The present invention relates to an evaporative emission control system for adsorbing fuel evaporated from a fuel tank, etc. (hereinafter referred to as an evaporative emission) of a vehicle such as a car and preventing the evaporative emission from being emitted to the atmosphere.
2. Prior Art
It is conventionally well known that the evaporative emission generated in the fuel tank, etc., of a vehicle such as a car is once adsorbed by an activated carbon in a canister so as to prevent it from being emitted in the atmosphere while the air is introduced into the canister by negative pressure generated in the engine of the car during the traveling of the car to thereby desorb the evaporative emission which was adsorbed by the activated carbon so that the desorbed or purged evaporative emission is supplied into the engine by way of an intake pipe and combusted in the engine.
However, in the tendency of a recent interest in a world wide level of an earth environmental problem, restriction for preventing the fuel evaporative emission generated in the vehicle from being emitted into the atmosphere is further imposed. Under the circumstances, the canister needs to have a sufficient adsorbing and desorbing properties for catching a large amount of evaporative emission generated in the car as the restriction is further imposed. To meet the need, merely the capacity of an adsorbent must be increased in the structure of the prior art canister, which results in making the canister inevitably large. It is difficult to mount such a large canister on the car. There are required such factors as the performance of the canister, namely, first is such an adsorbing performance that the adsorbent can adsorb the evaporative emission as much as possible, second is such a recycling performance that the adsorbent which adsorbed much evaporative emission can be recycled as much as possible. However, the adsorbent which has been currently used now can not desorb all the ingredients of the evaporative emission while it adsorbs and desorbs the evaporative emission repeatedly so that it accumulates the non-desorbed ingredients in its porosities, which results in the lowering of the canister performance. The adsorbents can be filled in the canister as much as possible taking into account the lowering of the canister performance, which makes the canister large.
If the amount of ingredients of the evaporative emission which is not desorbed and remained on the adsorbents (hereinafter referred to as remaining amount) can be reduced as much as possible, the amount of adsorbents to be filled in the canister can be reduced, which makes the canister small. As factors which influence the remaining amount, there are amount of vapor generated in the vehicle, ingredients of the evaporative emission which varies depending on the adsorbent properties and amount of air when the evaporative emission is purged. Aiming to the ingredients of the evaporative emission of these factors, the low boiling ingredient of the evaporative emission can be relatively easily desorbed even if it is adsorbed by the adsorbents but the high boiling ingredient is difficult to be desorbed, hence remains on the adsorbents. Furthermore, if a liquid fuel is stuck to the adsorbents, the adsorbents are soon deteriorated. To prevent the adsorbents from being deteriorated, it is proposed to provide a vapor-liquid separator as disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 63-104659 and No. 1-131858 in which the high boiling ingredient is liquefied upstream the adsorbents and only the low boiling ingredient is adsorbed by the adsorbents.
In this proposal, since vapor-liquid separation is performed by a mere bending of fluid, the fuel can be separated into liquid particles to same extent but the high boiling ingredient in the evaporative gas can not be sufficiently separated and liquefied. Since the separated liquid fuel is evaporated by a heater or returned to a fuel tank, there are required additional heating apparatus and thermal energies and additional piping for returning the liquid fuel to the fuel tank.